The present invention, in some embodiments thereof, relates to electrochemistry and more particularly, but not exclusively, to compositions comprising an alkali metal alloy which can be used in an electrode of an alkali metal battery.
In alkali metal ion batteries, the alkali metal (usually lithium) undergoes insertion or intercalation into a suitable material, such as an alloy or graphite. The alkali metal enters the anode structure during charge and exits during discharge. The alkali metal anode is typically protected by a solid electrolyte interphase (SEI) comprising salts of the alkali metal [Peled, J Electrochem Soc 1979, 126:2047-2051].
Graphite has been widely used as anode material in lithium ion batteries. In the fully charged state, its potential is very close to that of metallic lithium. This leads to possible problems with high-rate charging, such as in regenerative braking of hybrid electric vehicles (HEVs), especially at low ambient temperatures. In such circumstances, the lithium may form dendrite (needle-like) deposits on the surface of the graphite anode, which may penetrate the separator, short the battery, melt and cause a thermal runaway situation. In addition, a graphite intercalation anode has low gravimetric and volumetric energy densities, about 370 ampere hours per kg and 770 ampere hours per liter, compared to 3,800 ampere hours per kg for pure lithium.
Aluminum, tin and silicon each form lithium-rich alloys with high melting points and energy densities, e.g., Li4.4Sn (992 ampere hours per kg) and Li4.4Si (3,600 ampere hours per kg). However, the very large volume changes during the lithium intercalation-de-intercalation processes (over 300% for a silicon anode) lead to fast anode disintegration which severely limits extended deep cycling, thereby restraining practical application of such anodes. Attempts to reduce the volume change and disintegration by coating the silicon particles with a thin carbon film have not solved this problem, as the silicon particles expand upon lithiation (battery charge) and break the carbon shell.
U.S. Patent Application Publication No. 2013/0344391 describes battery electrode compositions comprising core-shell composites with a shell substantially permeable to metal ions, an active material such as silicon for storing and releasing metal ions, and a collapsible core (e.g., porous carbon material) and/or an internal void for accommodating changes in volume of the active material.
Liu et al. [Nano Lett 2012, 12:3315-3321] describes silicon nanoparticles inside of thin, self supporting carbon shells with a rationally designed void space between the particles and the shell, for use in a silicon electrode for lithium ion batteries. Voids were generated by using HF etching to remove SiO2 from partially oxidized silicon inside the carbon shell.
Wu & Xu [Adv Mater 2010: 22:1516-1520] describe a method of producing hollow silica spheres incorporated with different particles independent of their diameters, geometry and composition. Lithium ion batteries are described as a potential application for such particles.
Additional background art includes U.S. Pat. No. 6,908,706; U.S. Patent Application Publication No. 2011/0052998; U.S. Patent Application Publication No. 2013/0065128; U.S. Patent Application Publication No. 2013/0164620; Guo et al. [JACS 2013, 135:763-767]; Ji et al. [Nature Mater 2009, 8:500-506]; Peled et al. [J Electrochem Soc 1989, 136:1621-1625]; Seh et al. [Nature Commun 2013, 4:1331]; and Yamin et al. [J Electrochem Soc 1988, 135:1045-1048].